Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-walled carbon nanotubes (MWNTs), were discovered by lijima in 1991 (lijima, Nature, 1991, 354, 56). Subsequent to this discovery, single-walled carbon nanotubes (SWNTs), comprising a single graphene rolled up on itself, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals [lijima et al., Nature, 1993, 363, 603; and Bethune et al., Nature, 1993, 363, 605).
The seamless graphitic structure of single-walled carbon nanotubes (SWNTs) endows these materials with exceptional mechanical properties: Young's modulus in the low TPa range and (estimated) tensile strengths in excess of 37 GPa (Treacy et al., Nature 1996, 381, 678; Ruoff et al., Carbon 1995, 33, 925; Yakobson et al., Phys. Rev. Lett. 1996, 76, 2411; Lourie et al., J. Mater. Res. 1998, 13, 2418; lijima et al., J. Chem. Phys. 1996, 104, 2089; Cornwell et al., Solid State Comm. 1997, 101, 555; Lu, Phys. Rev. Lett. 1997, 79, 1297; Saito et al. Physical Properties of Carbon Nanotubes, Imperial College Press: London (1998); Yu et al., Phys. Rev. Lett. 2000, 84, 5552). Electron microscopy studies of SWNTs have shown that the nanotubes, although extremely strong in tension, are very flexible in bending (Lourie et al. Phys. Rev. Lett. 1998, 81, 1638; Vigolo et al., Science 2000, 290, 1331). Consequently, one would expect that incorporation of SWNTs as reinforcement in polymeric matrices could generate composites with greatly enhanced strength and toughness. To achieve this goal, the composites must possess sufficient structural continuity, so that external loads, imposed on the composite can be efficiently shared by the soft polymer matrix and the ultra high-strength nanotubes.
Several investigators have prepared a variety of composites, by embedding SWNTs in epoxy resins and other polymer matrices (Lozano et al., J. Appl. Polym. Sci. 2001, 79, 125; Lozano et al., J. Appl. Polym. Sci. 2001, 80, 1162; Schadler et al., Appl. Phys. Lett. 1998, 73, 3842; Ajayan et al., Adv. Mater. 2000, 12, 750). In most cases, the resulting composites have shown unremarkable mechanical properties and poor polymer-nanotube adhesion. The composites fractured at stresses, comparable to those of the non-reinforced polymer, with intact nanotubes pulling out from the matrix of either fragment. In all of these preparations the nanotubes were present in the matrix as discrete entities or small bundles. Hence, structural continuity within the composite depended entirely on adhesive (secondary) bonds between individual nanotubes and polymer chains. Given the marked difference in interfacial free energy between carbon nanotubes and organic macromolecules, it is not surprising that the adhesive bonds between these two entities are poor and the presence of discrete nanotubes does not strengthen the composite.